Conductive via composition

ABSTRACT

A conductive composition for filling a via. The composition is based on total composition and has 4.0-12.0 wt. % organic vehicle and 88.0-96.0 wt. % electrically conductive particles selected from the group consisting of silver and nickel and mixtures thereof.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention generally relates to conductive compositions. Inparticular, the invention is directed to compositions, which aresuitable for making conductive vias in a ceramic substrate.

[0003] 2. Description of the Related Art

[0004] Electrically conductive compositions have numerous applications.Thick film conductive compositions are screenable pastes, which are usedto form conductive elements in electronic applications. Suchcompositions contain electrically conductive material dispersed in ascreening agent.

[0005] Many electronic packages utilize a via to connect one level orlayer of circuitry to another. A via is a hole or aperture that isfilled with a conductive compound to provide an electrical connectionbetween different planes of a package allowing one side to beelectrically connected to another.

[0006] The via after processing needs to be very flat in order forsubsequent processing steps to succeed. The ends of the via need to beflat and not have a protrusion or a depression. The via also needs toadhere to the side walls of the via hole and not pull away from it afterprocessing.

[0007] Prior art via fill compositions have suffered from excessiveshrinkage during processing that cause depressions to form in the end ofthe via. The prior art compositions also can cause the via fill materialto pull away from the side wall of the via hole resulting in an opencircuit.

[0008] Prior art via fill compositions use silver and gold as themetallic component of the via fill. Gold is too expensive for mostapplications and therefore has limited use. Silver is preferred. Theproblem with silver is that it sinters at very low temperatures. Themore sintering that occurs, the more its volume decreases. Most viafills are fired at 850° C. which is close to silver's melting point of960° C. and therefore undergo substantial sintering. This causesshrinking in the via fill during firing which can result in a depressionon the surface and pulling away from the via side wall. Using materialsthat block sintering such Rhodium can reduce the problem. Rhodium isexpensive and does not completely block the sintering process. Nonconductive materials such as alumina can be added to the silver tohinder sintering. When added in large quantities (replacing 60% of thesilver) it does stop the sintering, however it also causes theelectrical conductivity of the via fill to increase drastically. Thishigh electrical resistance is detrimental to high frequency circuits.

[0009] A current unmet need exists for a conductive via composition thatovercomes the deficiencies of the prior art compositions.

SUMMARY

[0010] The present invention is a conductive composition for filling avia. The composition is based on total composition and includes:

[0011] a) 4.0-12.0 wt. % organic vehicle; and

[0012] b) 88.0-96.0 wt. % electrically conductive particles selectedfrom the group consisting of silver and nickel and mixtures thereof.

DETAILED DESCRIPTION

[0013] 1. Organic Components

[0014] The organic component is common to the thick film industry andare commonly called screening agents. The screening agent wets the metalparticles to form a paste that can be screened through a wire meshpattern. The organic components used in the present invention comprises4.0-12.0 wt. % of an organic vehicle or carrier based on totalcomposition. One such organic vehicle contains 2.0-6.0 wt. % pine oil,1.6-4.8 wt. % benzyl alcohol and 0.4-1.2 wt. % ethyl cellulose. Pine oilis commercially available from Chem Central Corporation. Benzyl alcoholis commercially available from Aldrich Chemical Corporation. Ethylcellulose is commercially available from Hercules Corporation.

[0015] In the conductive via composition of the present invention, theorganic vehicle is used in the range of 4.0-12.0 wt. % with a morepreferred range of 7.0-10.0 wt. %. If less than 4.0 wt. % is used, theresulting conductive via composition is difficult to place into a holeor has poor hole filling properties. If more than 12.0 wt. % is used,the resulting via tends to have dimples on the ends of the via and pullsaway from the via hole side wall. Other organic vehicles could also beused.

[0016] 2. Conductive Component:

[0017] The electrically conductive component of the present inventioncomprises fine spheroid shaped particles of electrically conductivematerials such as silver, nickel, and mixtures thereof. A mixture ofnickel and silver is the preferred conductive component. The conductiveparticles comprise 88.0 to 96.0 wt. % of the total composition with apreferred range of 90.0 to 93.0 wt. %.

[0018] The mixture of nickel and silver comprises 15.0 to 60.0 wt %silver and 28.0 to 81.0 wt % nickel of the total composition. A morepreferred range of the nickel silver mixture is 15.0 to 30.0 wt. %silver and 66.0 to 78.0 wt. % nickel.

[0019] The preferred silver spheres are commercially available fromShoei Chemical Corporation as product number A-128. Silver spheres witha diameter in the range of 1.0-5.0 microns are used with a preferredsize of 2.5 microns. The silver particles need to be spherical in orderto promote flow into the via.

[0020] The preferred nickel spheres are commercially available fromAtlantic Equipment Engineers as product number Ni-112. Nickel sphereswith a diameter in the range of 15.0-25.0 microns are used with apreferred size of less than 20.0 microns. The nickel particles need tobe spherical in order to promote flow into the via. Larger nickelspheres clog the mask or screen used to fill the via. Smaller nickelspheres have too much surface area for the silver to coat.

[0021] 3. General Composition Preparation and Printing Procedures

[0022] In the preparation of the composition of the present invention,the electrically conductive metallic particles are mixed with theorganic components. The organic vehicle, nickel spheres and silverspheres are placed in a three-roll mill and mixed for 20 minutes to forma paste. Moderate pressure is applied to the rolls. The resulting pastehas a viscosity of 275 to 375 thousand centipoise.

[0023] Three-roll mill mixing is preferred for preparing the via fillcomposition. The via fill composition is placed in a bladder via fillmachine manufactured by Pacific Trinetics Corporation. The bladder viafill machine has a mask with holes that correspond to the vias to befilled. A ceramic alumina substrate having holes to be filled is placedunder the mask. The via fill composition is pressurized and extruded bythe bladder fill machine through the mask and into the via. Screenprinting can also be used to fill the vias. The vias are typically 8mils in diameter and can range from 4 to 10 mils in diameter. Thesubstrate is preferably 96% alumina and can range from 10 to 40 milsthick. Once the via is filled and fired it is no more than 25 micronsabove or below the top and bottom plain of the substrate.

[0024] The alumina substrate with filled vias is then cured in an ovenat a temperature range of 80 degrees Celsius for 20 minutes. Thesubstrate is then fired in an oven to sinter the via fill composition.The firing is done in an air atmosphere oven at 800 to 900 degreesCelsius for 5 to 30 minutes. Typical fired vias had a resistance of 0.05ohm per via for a 8 mil diameter via that is 40 mils long.

[0025] 4. Remarks

[0026] The nickel in the composition minimizes the sintering of thesilver and also helps to maintains conductivity. The nickel does notsinter appreciably at the 850° C. firing temperature since its meltingpoint is 1455° C. Also, as the silver tries to alloy with the nickel themajority of the silver nickel alloys also melt at a high temperature ofapproximately 1435° C. Nickel is also a fair conductor of electricity soits use as a partial replacement for silver does not significantlyeffect high frequency performance. The conductivities of several metalsare given below: Metal Conductivity (Microhm-Cm) Silver 1.59 Copper 1.67Gold 2.35 Nickel 6.84 Iron 9.71 Tin 11.0 Lead 20.64

[0027] Changing the Nickel/Silver ratio has the following effect on thefired dimensions of the filled via. Starting with 100% Nickel the viaswill conduct electricity and be very flat on the surface with little orno dimples. Note: dimples and protrusions are also effected by theorganic vehicle percentage and will be discussed later. However aftermultiple firings, such as would be experienced to produce a thick filmresistor circuit, the via will crack causing an open circuit and nolonger conduct electricity. As silver is added to the composition, noimprovement was observed until the silver ratio reaches about 15% of thetotal metal loading. At this point, the vias that were produced werestill flat and continued to conduct electricity after the multiplefirings. As even more silver is added, there was no change in theelectrical performance after multiple firings but the vias begin todimple. The dimples continue to get deeper until at around 60% silverloading, they are about one mil below the surface of the substrate. Thisis caused by the silver sintering as is found in the prior art.

[0028] The viscosity value of the resulting paste does not completelydescribe the flow properties of the invention. Actual via filling isrequired to fully evaluate the material. Slight adjustments are made inthe organic percentage to adjust for dimples or protrusions in thefilled via. The via fill of the invention is a slurry and behavesaccordingly. As it becomes thick (high viscosity) it does not flow andtherefore does not level on via filling and produces protrusions. Addingmore organic reduces protrusions. As more organic is added the via fillmaterial becomes thin (low viscosity) and flows after via filling toproduce a flat surface. As more organic is added dimples begin to form.This is because the organic now makes up a substantial part of the viafill material and it is lost during firing as it evaporates. Thischanges the volume of the material in the filled via and causes it toshrink which produces the dimples on the surface.

[0029] 5. Test Procedures

[0030] Side Wall Adhesion:

[0031] The adhesion of the via fill composition within the hole wasvisually checked with a scanning electron microscope aftercross-sectioning. Any air gaps between the via fill composition and theside wall of the hole would show loss of side wall adhesion. Passcriteria is no air gaps.

[0032] Mechanical Dimensions of Dimples and Protrusions:

[0033] Mechanical dimensions of the fired vias were measured by amicroscope. The maximum height of any protrusions and maximum depth ofany dimples was measured.

EXAMPLES

[0034] The present invention will be described in further detail bygiving practical examples. The scope of the present invention, however,is not limited in any way by these practical examples.

Example 1

[0035] This example describes the preparation of a via fill compositionusing nickel spheres with an average diameter of 20 microns and silverspheres with an average diameter of 5 microns. The components below wereadded to a 50-ml jar with mixing. The mixture was then roller milled ina three-roll mill for 20 minutes. Test results are shown in table 1.Component Weight (%) Pine oil 2.9 Benzyl alcohol 2.8 Ethyl cellulose 0.6Silver spheres 18.7 Nickel spheres 75.0

Example 2

[0036] This example describes the preparation of a via fill compositionusing nickel spheres with an average diameter of 20 microns and silverspheres with an average diameter of 5 microns. The components below wereadded to a 50-ml jar with mixing. The mixture was then roller milled ina three-roll mill for 20 minutes. Test results are shown in table 1.Component Weight (%) Pine oil 3.2 Benzyl alcohol 3.1 Ethyl cellulose 0.6Silver spheres 18.6 Nickel spheres 74.5

Example 3

[0037] This example describes the preparation of a via fill compositionusing nickel spheres with an average diameter of 20 microns and silverspheres with an average diameter of 5 microns. The components below wereadded to a 50-ml jar with mixing. The mixture was then roller milled ina three-roll mill for 20 minutes. Test results are shown in table 1.Component Weight (%) Pine oil 3.7 Benzyl alcohol 3.5 Ethyl cellulose 0.7Silver spheres 18.4 Nickel spheres 73.7

[0038] TABLE 1 Properties Example 1 Example 2 Example 3 Side WallAdhesion Pass Pass Pass Dimples (microns)  0  5 25 Protrusions (microns)50 20  0

[0039] While the invention has been taught with specific reference tothese embodiments, someone skilled in the art will recognize thatchanges can be made in form and detail without departing from the spiritand the scope of the invention. The described embodiments are to beconsidered in all respects only as illustrative and not restrictive. Thescope of the invention is, therefore, indicated by the appended claimsrather than by the foregoing description. All changes that come withinthe meaning and range of equivalency of the claims are to be embracedwithin their scope.

What is claimed is:
 1. A conductive composition for filling a via, based on total composition comprising: a) 4.0-12.0 wt. % organic vehicle; and b) 88.0-96.0 wt. % electrically conductive particles selected from the group consisting of silver and nickel and mixtures thereof.
 2. The conductive composition according to claim 1, wherein the electrically conductive particles comprise 90.0 to 93.0 wt. % of the total composition.
 3. The conductive composition according to claim 1, wherein the organic vehicle based on total composition comprises: a) 2.0-6.0 wt. % pine oil; b) 1.6-4.8 wt. % benzyl alcohol; and c) 0.4-1.2 wt. % ethyl cellulose.
 4. The conductive composition according to claim 3, wherein the electrically conductive particles based on total composition further comprise: a) 15.0-60.0 wt. % silver; and b) 28.0-81.0 wt. % nickel.
 5. The conductive composition according to claim 4, wherein the electrically conductive particles based on total composition further comprise: a) 15.0-30.0 wt. % silver; and b) 66.0-78.0 wt. % nickel.
 6. The conductive composition according to claim 5, wherein the electrically conductive particles have a spherical shape.
 7. The conductive composition according to claim 6, wherein the nickel particles have a diameter of 15.0-25.0 microns.
 8. The conductive composition according to claim 6, wherein the silver particles have a diameter of 1.0-5.0 microns.
 9. The conductive composition according to claim 1, wherein the conductive composition is applied to a substrate that is chosen from the group consisting of alumina ceramic and aluminum nitride.
 10. The conductive composition according to claim 9, wherein the conductive composition is cured at a temperature range from 800 degrees Celsius to 900 degrees Celsius.
 11. The conductive composition according to claim 10, wherein the conductive composition has a cure time between 5 and 30 minutes.
 12. A conductive via fill composition, based on total composition comprising: a) 4.0-12.0 wt. % organic vehicle; and b) 15.0-60.0 wt. % silver particles; and c) 70.0-78.0 wt. % nickel particles.
 13. The conductive via fill composition according to claim 12, wherein the organic vehicle based on total composition comprises: a) 2.0-6.0 wt. % pine oil; b) 1.6-4.8 wt. % benzyl alcohol; and c) 0.4-1.2 wt. % ethyl cellulose.
 14. The conductive via fill composition according to claim 13, further comprising: a) 15.0-30.0 wt. % silver; and b) 66.0-78.0 wt. % nickel.
 15. The conductive via fill composition according to claim 14, wherein the nickel and silver particles have a spherical shape.
 16. The conductive via fill composition according to claim 15, wherein the nickel particles have a diameter of 15.0-25.0 microns.
 17. The conductive via fill composition according to claim 16, wherein the silver particles have a diameter of 1.0-5.0 microns.
 18. The conductive via fill composition according to claim 12, wherein the conductive composition is applied to a alumina ceramic.
 19. The conductive via fill composition according to claim 18, wherein the composition is cured at a temperature from 800 degrees Celsius to 900 degrees Celsius.
 20. The conductive via fill composition according to claim 19, wherein the composition has a cure time between 5 and 30 minutes. 